1. Field of the Invention
The present invention relates to an improved process for producing allyl bromides useful in the field of synthetic organic chemistry, in particular, as intermediates for producing fine chemical products, such as agricultural chemicals, medicines, dyes, etc.
2. Description of the Prior Art
Various processes have hitherto been disclosed with regard to the production of allyl bromides represented by Formula (I) of the following: ##STR1## (wherein R, R.sup.1 and R.sup.2 each independently represents a hydrogen atom or a lower alkyl group).
Of allyl bromides represented by Formula (I), allyl bromide (R.sup.1 =R.sup.2 =H),/the basic compound represented by the formula, is generally produced by reacting allyl alcohol with hydrobromic acid in the presence of sulfuric acid. In Organic Synthesis, Coll. Vol. 1, p.27 (1967), for example, 5.9 mol of 48% hydrobromic acid is admixed with 4 mol of allyl alcohol, and 300 g of concentrated sulfuric acid is gradually added with stirring to the mixture, followed by the distillation thereof to obtain allyl bromide. By this method, the desired allyl bromide can be produced almost quantitatively in a yield of 92 to 96%. However, the method is disadvantageous as an industrial process since it utilizes a large quantity of sulfuric acid and hence results in the generation of a large quantity of waste which must be subjected to costly treatments, including neutralization.
There is also known a process in which propylene is brominated with hydrogen bromide at an elevated temperature (350.degree. to 450.degree. C.) in the presence of a pearlite catalyst to produce allyl bromide [Soviet Patent No. 753,841 (1980)]. In this process, however, 1-bromopropene is produced as a by-product, and allyl bromide is formed with only a low selectivity. In addition, the by-product could not be separated without difficulty.
It is also known that allyl bromides can be produced from allyl chlorides through halogen exchange. For example, in Journal of the Organic Chemistry USSR, Vol. 10, p. 1,122 (1974), allyl chloride or methallyl chloride is subjected to halogen exchange with an excess of hydrobromic acid in the presence of cuprous chloride to produce allyl bromide or methallyl bromide.
In Journal of the American Chemical Society, Vol. 72, p. 4,316 (1950) is disclosed a process in which methallyl chloride is subjected to halogen exchange with sodium bromide in methanol to give methallyl bromide, as well as a process in which methallyl chloride is treated with lithium bromide in acetone to give methallyl bromide.
In the former process, however, methallyl chloride can be converted at a conversion rate of only 70% or less, and the catalyst used contains heavy metals which have been placed under severe industrial regulation and hence requires a particular caution for the treatment of waste water contaminated with them. In the latter process, the halogen exchange in methanol gives methallyl bromide in only an extremely low yield due to the formation of large quantities of by-products, such as methallyl methyl ether, etc., and the halogen exchange in acetone gives methallyl bromide in a yield of only 54% at best.
Although a number of other processes are known for the production of allyl bromides, they are not satisfactory in yield, materials employed, and the like.
It is therefore the present state of the art that none of the hitherto known processes for producing allyl bromides is satisfactory in overall with regard to yield, process, conditions, and the like.
Taking into consideration the present state of the art described hereinabove, the present inventors have conducted intensive investigations to establish an effective, commercially advantageous process for producing allyl bromides from allyl chlorides.
It is known that the exchange of halogen between halogenated alkyls is in general an equilibrium reaction. The conversion of allyl chlorides into allyl bromides according to the present invention is also an equilibrium reaction. It is therefore theoretically possible to continuously draw the reaction product (allyl bromides) out of the reaction system, or to use the halogen exchanging agent (allyl chlorides) in large excess, so as to allow the conversion to proceed in a favorable manner. However, when the former technique is applied to the conversion, it can be extremely difficult to selectively separate allyl bromides alone during the course of the reaction since both the raw material (allyl chlorides) and the product (allyl bromides) are liquid and are highly soluble in ordinary organic solvents and, in addition, they have similar physical properties. The latter technique has a difficulty in dissolving metal bromides in large quantities, in the recovery and recycling of metal bromides, and in the separation of metal chlorides generated as by-products.